1. Field of the Invention
The present invention relates to a charged-particle apparatus with a plurality of charged-particle beams. More particularly, it relates to an apparatus which employs plural charged-particle beams to simultaneously acquire images of plural scanned regions of an observed area on a sample surface. Hence, the apparatus can be used to inspect and/or review defects on wafers/masks with high resolution and high throughput in semiconductor manufacturing industry.
2. Description of the Prior Art
For manufacturing semiconductor IC chips, pattern defects and/or uninvited particles (residuals) inevitably appear on a wafer and/or a mask during fabrication processes, which reduce the yield to a great degree. To meet the more and more advanced requirements on performance of IC chips, the patterns with smaller and smaller critical feature dimensions have been adopted. Accordingly, the conventional yield management tools with optical beam gradually become incompetent due to diffraction effect, and yield management tools with electron beam are more and more employed. Compared to a photon beam, an electron beam has a shorter wavelength and thereby possibly offering superior spatial resolution. Currently, the yield management tools with electron beam employ the principle of scanning electron microscope (SEM) with a single electron beam, which therefore can provide higher resolution but can not provide throughputs competent for mass production. Although a higher and higher current of the single electron beam can be used to increase the throughputs, the superior spatial resolutions will be fundamentally deteriorated by the Coulomb Effect which increases with the beam current.
For mitigating the limitation on throughput, instead of using a single electron beam with a large current, a promising solution is to use a plurality of electron beams each with a small current. The plurality of electron beams forms a plurality of probe spots on one being-inspected or observed surface of a sample. The plurality of probe spots can respectively and simultaneously scan a plurality of small scanned regions within a large observed area on the sample surface. The electrons of each probe spot generate secondary electrons from the sample surface where they land on. The secondary electrons comprise slow secondary electrons (energies ≤50 eV) and backscattered electrons (energies close to landing energies of the electrons). The secondary electrons from the plurality of small scanned regions can be respectively and simultaneously collected by a plurality of electron detectors. Consequently, the image of the large observed area including all of the small scanned regions can be obtained much faster than that scanned with a single beam.
The plurality of electron beams can be either from a plurality of electron sources respectively, or from a single electron source. For the former, the plurality of electron beams is usually focused onto and scans the plurality of small scanned regions within a plurality of columns respectively, and the secondary electrons from each scanned region are detected by one electron detector inside the corresponding column. The apparatus therefore is generally called as a multi-column apparatus. The plural columns can be either independent or share a multi-axis magnetic or electromagnetic-compound objective lens (such as U.S. Pat. No. 8,294,095). On the sample surface, the beam interval between two adjacent beams is usually as large as 30˜50 mm.
For the latter, a source-conversion unit virtually changes the single electron source into a plurality of sub-sources. The source-conversion unit comprises one beamlet-forming (or beamlet-limit) means with a plurality of beam-limit openings and one image-forming means with a plurality of electron optics elements. The plurality of beam-limit openings divides the primary-electron beam generated by the single electron source into a plurality of sub-beams or beamlets respectively, and the plurality of electron optics elements influence the plurality of beamlets to form a plurality of parallel (virtual or real) images of the single electron source. Each image can be taken as one sub-source which emits one corresponding beamlet. To make more beamlets available, the beamlet intervals are at micro meter level. Naturally, one primary projection imaging system and one deflection scanning unit within one single column are used to project the plurality of parallel images onto and scan the plurality of small scanned regions respectively. The plurality of secondary electron beams therefrom is directed by one beam separator into one secondary projection imaging system, and then focused by the secondary projection imaging system to be respectively detected by a plurality of detection elements of one electron detection device inside the single column. The plurality of detection elements can be a plurality of electron detectors placed side by side or a plurality of pixels of one electron detector. The apparatus therefore is generally called as a multi-beam apparatus.
The beamlet-forming (or beamlet-limit) means is usually an electric-conduction plate with through-holes, and a plurality of through-holes therein function the plurality of beam-limit openings respectively. Two methods have been used to form the plurality of parallel images by the image-forming means. For the first one, each electron optics element has an electrostatic micro-lens which focuses one beamlet and therefore forms one real image, such as U.S. Pat. No. 7,244,949. For the second one, each electron optics element has a electrostatic micro-deflector which deflects one beamlet and therefore forms one virtual image, such as U.S. Pat. No. 6,943,349 and first cross reference. The concept of using an electrostatic deflector to form a virtual image of an electron source was used in the famous two-slit electron interference experiments as early as in 1950s (FIG. 1 of the paper “The Merli-Missiroli-Pozzi Two-Slit Electron-Interference Experiment” published in Physics in Perspective, 14 (2012) 178-195 by Rodolfo Rosa). Coulomb Effect in the second method is weaker than in the first method due to one real image has a higher current density, and hence the second method is more advantageous for achieving both high throughput and high resolution.
To reduce aberrations of the plurality of probe spots, the primary projection imaging system basically comprises one transfer lens and one objective lens, and the transfer lens bends the plurality of beamlets to pass through the objective lens as close to the optical axis thereof as possible. For a source-conversion unit with the second method, the bending function of the transfer lens can be done by the plurality of micro-deflectors, and therefore the transfer lens can be removed, as proposed in the first cross reference and shown in FIG. 1. Without the transfer lens, the projection imaging system will be simplified and easy in manufacturing and operation.
In FIG. 1, the electron source 101 on the primary optical axis 100_1 generates the primary electron beam 102 with a source crossover (virtual or real) 101s. The condenser lens 110 focuses the primary-electron beam 102 incident onto the source-conversion unit 120 with a desired current density. The peripheral electrons of the primary electron beam 102 are cut off by the main opening of the main aperture plate 171, which can also be placed above the condenser lens no. Three beamlets (102_1, 102_2 and 102_3) of the primary-electron beam 102 are respectively deflected towards the primary optical axis 100_1 by the three micro-deflectors (122_1, 122_2 and 122_3) of the image-forming mean 122, and pass through three beam-limit openings (121_1, 121_2 and 121_3) of the beamlet-limit means 121. Then, the three virtual images (102_1v, 102_2v and 102_3v) formed by the deflected three beamlets are projected by the objective lens 131 onto the surface 7 of the sample 8 and three probe spots (102_1s, 102_2s and 102_3s) therefore are formed thereon.
If the three beamlets are deflected close to or passing through the front focal point of the objective lens 131, they will perpendicularly land on the sample surface 7 and aberrations of the off-axis probe spots (such as 102_2s) due to the objective lens 131 will decrease to a great degree. However, in this case, the deflection angles of the three beamlets become larger, which not only require stronger deflection powers of the three micro-deflectors but also generate larger deflection aberrations. The first issue may incur electric breakdown of the three micro-deflectors, and the second issue may enlarge the sizes of the off-axis probe spots to an unacceptable level.
The beam-limit openings limit the currents of the three probe spots, and the currents are changed by tuning the focusing power of the condenser lens 110 to vary the current density of the primary electron beam 102. For the three micro-deflectors, the incident angles of the three beamlets change with the focusing power, and the deflection powers thereof need adjusting accordingly. The time and the effort for changing observing conditions are the less the better.
Accordingly, it is necessary to provide a multi-beam apparatus which has no or fewer foregoing issues, and therefore can provide high image resolution and high throughput. Especially, a multi-beam apparatus which can inspect and/or review defects on wafers/masks with high resolution and high throughput is needed to match the roadmap of the semiconductor manufacturing industry.